Abstract

The deformation of continental crust during continental collision by folding and
thrusting follows three types of structural styles: (1) in a true thin-skinned style only
cover rocks are involved; (2) in the case of a thin-skinned basement-involved style, thin
slabs of crystalline basement rocks are piled up into a nappe stack; (3) in a true
thick-skinned style, the entire upper crust is involved in the deformation. In the Alps
all three styles can be recognized. The Helvetic nappes and parts of the Penninic nappes
exhibit true thin-skinned style tectonics. Triassic evaporites, Jurassic shales and
Cretaceous marls acted as detachment horizons. Basement-involved thin-skinned tectonics is
typical for the Penninic nappes in the core of the orogen. The thickness of the basement
thrust sheets is controlled by the effects of Mesozoic rifting, by deep burial and heating
of the subducting crust and by the presence of Late Palaeozoic structures. Thick-skinned
style is observed in the more external parts of the orogen, the external massifs and the
Southalpine nappe system. It occurred in the late phase of collision and involved the
entire upper crust. The basal detachment occurred possibly along phyllonites generated by
the breakdown of load-bearing feldspar. Considering the Alpine orogen as a whole, the
lower crust deformed seemingly independently from the upper crust. The detachment of the
cover units by thin-skinned tectonics occurred prior to thrusting related to
basement-involved thin-skinned tectonics. Thrust faults of both types were overprinted by
‘post-nappe folding’.